The present invention generally relates to the fabrication process of semiconductor devices, and more particularly to a process for cleaning a semiconductor substrate by a cleaning liquid.
With the increasing integration density and decreasing pattern width in the recent integrated circuits, the problem of deposition of minute silicon oxide particles or silicon particles on the semiconductor substrate during the fabrication process of the semiconductor devices is becoming an acute problem. When these particles are deposited on a semiconductor substrate, various problems occur particularly in the step of photolithographic exposure and patterning. The silicon oxide particles and the silicon particles are formed as a result of the fabrication process of the semiconductor substrates. In order to achieve a high yield and hence low fabrication cost of semiconductor devices, it is essential to remove these particles from the surface of the semiconductor substrate before starting the fabrication process of the semiconductor device.
Conventionally, the use of a cleaning liquid comprising a mixture of ammonia water (aqueous ammonia), hydrogen peroxide aqueous solution and deionized water is known for this purpose. Thereby, ammonia acts as a mild etchant for separating the particles that are bonded to the surface of the substrate, while hydrogen peroxide is believed to act as a mild oxidant for compensating the etching action of ammonia. Typically, ammonia water of 29 wt % composition for industrial use and hydrogen peroxide aqueous solution of 31 wt % composition also for industrial use are mixed with deionized water with a volume ratio of 1:1:5.
Next, a conventional process for removing the particles from the silicon substrate by the foregoing cleaning liquid will be described for the case of a p-type silicon wafer of four inch diameter and a resistivity of 10 .OMEGA.cm. The silicon wafer has a (100) oriented crystal surface. In some cases, a silicon oxide layer may be formed on the surface by a thermal oxidation process that is typically conducted at 950.degree. C. in a dry oxygen environment.
When removing the particles of silicon oxide or silicon from the surface, the silicon wafer is dipped in the foregoing liquid mixture of ammonia water, hydrogen peroxide aqueous solution and deionized water held at about 80.degree. C. for 10 minutes. After pulling out from the liquid mixture, the wafer is rinsed by deionized water for 10 minutes and dried subsequently for 15 minutes. As a result of the dipping into the liquid mixture, the surface of the substrate is etched slightly and the particles deposited on the surface of the substrate are separated from the surface.
In one experiment, it was found that the surface of the silicon oxide layer covering the surface of the silicon wafer is etched for a depth of 32 .ANG.. When the silicon wafer is not covered by the silicon oxide layer and the (100) crystal surface of silicon is exposed, on the other hand, the surface of the substrate was etched for a depth of about 75 .ANG.. The result of particle counting conducted for the particles that are remaining on the surface of the substrate and having the diameter of larger than about 0.3 .mu.m showed that about 94 percent of the silicon oxide particles are removed from any of the silicon oxide surface and the silicon surface, while the silicon particles are removed by only 36 percent.
When fabricating the integrated circuits having submicron patterns, the foregoing result of the removal of particles is not sufficient for both the silicon particles and silicon oxide particles. Further, the amount of etching that is made at the time of cleaning in the liquid mixture is generally too much for the submicron integrated circuits. In addition, the use of conventional liquid mixture for the cleaning liquid causes the problem of environmental pollution because of the large amount of waste ammonia formed at the time of processing. The use of large amount of hydrogen peroxide and ammonia water of course increases the cost of the fabricated integrated circuits.